Metrohm 791 VA Detector - Manual (Page 39)

4.3 Malfunctions and their rectification
791 VA Detector
33
Malfunction Cause Rectification
Noise level too high • Inadequate pulsation
damping.
• Electrostatic screening
defective.
• Background current too high.
• Eluent outlet tubing under
electrostatic influence.
• Defective working electrode.
• Defective 791 VA Detector.
• Use pulsation dampener.
• Check screening, if necessary
connect steel column and/or its
outlet to socket 14 of the VA
Detector by means of a cable.
• Check screening, if necessary
connect steel column and/or its
outlet to socket 14 of the VA
Detector by means of a cable.
• Immerse eluent outlet tubing
completely in spent solution in
waste recipient or renew PTFE
tubing if necessary.
• Replace working electrode.
• Check 791 VA Detector using
the dummy cell (see section
4.1.4)
Spikes on baseline • Air in detector cell.
• Air entering detector cell.
• If a CPE working electrode is
in use, the carbon paste is not
in good condition.
• Unscrew working electrode and
deaerate detector cell.
• Degas eluent or subject it to
appropriate treatment.
• Refill CPE and ensure paste is
well tamped down.
Isolated spikes on
baseline • Electrostatic discharges from
operating personnel on the
apparatus.
• Mains interference.
• Electromagnetic interference
from environment.
• Such discharges cannot be
completely avoided, but are not
usually too troublesome at
normal (i.e. not too dry) atmos-
pheric humidity.
• Connect all instruments to the
power supply via a suitable filter
(e.g. Metrohm 615 Mains Dis-
tributor).
• Change "environment".
Substance turnover
too low or non-
existent • Polarization voltage too low.
• Active surface of working
electrode blocked by sub-
stances.
• Increase polarization voltage.
• Polish working electrode or
renew paste in CPE.
Substance turnover
falls off • Active surface of working
electrode blocked by sub-
stances.
• Quantity of substance too
great.
• Polish working electrode or
renew paste in CPE.
• The sample quantity should be
less than 100 ng per injection to
avoid blocking of the electrode
surface.
Front peak too high • Environment altered by
sample solution.
• Eluent contaminated by Fe(II)
ions from rusty steel parts
(traces only suffice!).
• Whenever possible, use eluent
as solvent for the sample solu-
tion.
• Check entire HPLC or IC system
for rust; check resistance of all
materials to the reagents in use.